Urinary System - Anatomy & Physiology

Urinary organs

• Kidneys (2)

• Ureters (2)

• Urethra

• Urinary bladder

Kidney (location, structure)

• 2 kidneys located retroperitoneally on the posterior wall of abdominal cavity at about the level of T12-L3

  

• Parts

  • Renal capsule: connective tissue covering the kidney

  • Renal cortex: outer layer of the kidney, contains nephrons

  • Renal medulla: inner layer, contains striated regions called medullary pyramids

  • Renal pelvis: innermost part, collects urine before it travels down ureter

Ureters

• 2 ureters convey urine from kidney to bladder, about 25cm long, smooth muscle

Urethra

• Convey urine from bladder to outside, smooth muscle

• 4cm long in women

• 20cm long in men

• Internal urethral sphincter

• External urethral sphincter

Urinary bladder (layers)

4 layers

1. Innermost: transitional epithelium

2. Connective tissue w/ lots of elastic fibers

3. Smooth muscle layer w/ fibers running in all directions (detrusor muscle)

4. Serous coat, only covers the superior portion of the bladder (is continuous with the parietal peritoneum of abdominal cavity)

Nephron

• Functional unit of the kidney

• Each kidney contains roughly 1 million nephrons

• Nephron = renal tubule + its blood supply

  • Parts of renal tubule

    1. Bowman’s (glomerular) capsule

    2. Proximal convoluted tubule

    3. Loop of Henle

    4. Distal convoluted tubule

    5. Collecting duct

4 steps of urine production

1. Filtration

   • Fluids from blood collected

   • Includes water + solutes

2. Reabsorption

   • Selectively reabsorbs necessary substances back to blood

3. Secretion

   • Pump out unwanted substances to urine

4. Excretion

   • Remove excess substances and toxins from body

Nephron’s blood supply

• Glomerulus: knot of capillaries, under high pressure, where filtration occurs

  • Fed by afferent arteriole

  • Drained by efferent arteriole

  • Enclosed by Bowman’s (glomerular) capsule

  • Have a lot of fenestrae (openings) to make them highly permeable

  • Covered in cells called podocytes that make them impermeable to blood proteins

    • Everything except 2 things can enter filtrate: proteins and RBCs

• Peritubular capillaries: net of capillaries that surround the renal tubule, under low pressure

Kidney functions

• Regulates the composition and pH of body fluids

• Removes metabolic wastes from body

  • Urea: byproduct of amino acid metabolism

  • Uric acid: byproduct of nucleic acid metabolism

  • Creatinine: associated with creatine metabolism in muscles

• Helps control RBC (red blood cell) production (erythropoietin) 

• Helps activate vitamin D

• Helps maintain blood volume and pressure

Mneumonic for kidney functions

• A = maintains ACID-base balance

• W = maintaining WATER balance

• E = ELECTROLYTE balance

• T = TOXIN removal

• B = BLOOD pressure control

• E = making ERYTHROPOIETIN

  • Hormone made in kidneys - acts on RBC to protect from destruction and stimulates stem cells in bone marrow to produce RBC

• D = Vitamin D metabolism

3 steps of urine formation

• Filtration

• Reabsorption

• Secretion

  

Urine formation: filtration

• Passive, non-specific process

• Blood in glomerulus is under high pressure which forces fluid and small solutes out of the blood into Bowman’s capsule

• Product of filtration is FILTRATE

  • Filtrate = blood plasma (without plasma proteins because blood cells and proteins are too big to move out of the glomerulus)

• Process of filtration produces ~145 L of fluid per day (more than 4X total body water)

Why is blood in the glomerulus under high pressure?

Efferent arteriole has a smaller diameter than the afferent arteriole

4 parts/locations of reabsorption process

1. PCT (Proximal convoluted tubule)

2. LOH (Loop of Henle)

3. DCT (Distal convoluted tube)

4. CD (Collecting Duct)

What is reabsorption process

• The process that returns the “good stuff” back to the blood from the filtrate

  • PCT (proximal convoluted tubule)

    • 100% glucose, lactate, amino acids (active transport)

    • 90% K (active transport)

    • 70% sodium and H₂O (active transport)

    • 50% chloride (passive transport)

  • LOH (Loop of Henle)

    • More Na, Cl, and K resorbed by active transport

    • 15% more H₂O (descending limb)

  • DCT (distal convoluted tubule)

    • The rest of H₂O and Na could be reabsorbed if needed (aldosterone and ADH)

  • CD (collecting duct)

    • More H₂O reabsorption if needed (ADH)

Secretion

• A final “fine tuning” of the urine by moving substances from blood to urine

  • PCT: actively secretes many medications+drugs

  • H+ secreted throughout the entire renal tubule (important in blood pH regulation)

  • DCT and CD: some K+ secreted

Hormonal regulation of urine volume and concentration

• Aldosterone

• ADH

• Renin

• Angiotensin II

• ANF

Aldosterone

Stimulates DCT to reabsorb Na (and H₂O) and secrete K+

ADH

Increases DCT and CD permeability to H₂O (inserts aquaporins)

Renin

• Renin: enzyme released by juxtaglomerular cells when BP in afferent arteriole drops

  1. Renin reacts with a blood protein called angiotensinogen and converts it to angiotensin I

  2. ACE (angiotensin converting enzyme) in lungs and plasma converts angiotensin I to angiotensin II

Angiotensin II actions

1. Vasoconstricts efferent arteriole to raise glomerular pressure and increase glomerular filtration rate when BP is low

2. Makes adrenal cortex secrete aldosterone

3. General vasoconstrictor

ANF: Atrial natriuretic factor

• Secreted by heart when blood volume increases

• Inhibits ADH, aldosterone, and renin secretion

Normal urine composition

• Clear

• Pale to deep yellow color (urochrome = byproduct of Hb destruction)

• Sterile

• pH varies from 4.5-8 (varies w/ diet)

  • More acidic if diet is high in protein

  • More alkaline if diet is vegetarian or if you have a UTI

• Specific gravity of 1.001-1.035 (relative weight of specific volume of liquid compared with equal volume of water which has a specific gravity of 1.000

• Varying amounts of ions

• Metabolic waste: urea, uric acid, creatinine

• Water

Abnormal urine components

• Glucose (glycosuria)

• Albumin (albuminura)

• RBC (hematuria)

• WBC (pyuria)

• Hb

• Bile

• Ketone bodies

• Casts

• Crystals

Glucose

Glycosuria

• Diabetes

Albumin

Albuminuria

• Excessive exercise

• Physical trauma to kidney

• Glomerulonephritis

RBC

Hematuria

• Physical trauma to kidney

• Infection

• Kidney stones

WBC

Pyuria

• Infection

Hb

Indicates increased breakdown of RBC perhaps due to anemia or autoimmune disease

Bile

• Hepatitis

• Cirrhosis

Ketone bodies

Ketonuria

• Starvation

• Out of control diabetes

• SUPER low carb diet (ketogenic diet)

Casts

 hardened cell fragments formed in DCT and CD that get flushed out of the urinary tract

• WBC casts, RBC casts, waxy casts, granular casts

Crystals

Micturition reflex

• Filling of bladder stimulates stretch receptors

• Afferent impulses travel to micturition reflex center in spinal cord

• Efferent impulses travel to detrusor muscle which contracts rhythmically (giving the sensation of having to pee)

• Reflex starts at about 150ml, more powerful contractions the more full it gets

• Internal urethral sphincter (smooth muscle, involuntary)

• External urethral sphincter (skeletal muscle, voluntary)

Extra info on micturition (urination)

As the bladder fills, the smooth muscle layer relaxes so no pressure builds in the organ, until a certain volume is reached (~150/200ml) and then the muscle can’t relax any more and pressure increases, stretch receptors are stimulated, triggering the reflex.  

“Urgent” volume is ~450 ml

Avg person voids about 1.5 L urine per day.